In the production of hydrocarbons from a hydrocarbon-bearing formation there is normally provided a well which extends from the surface of the earth into the formation. The hydrocarbon-bearing portion of the formation may be overlain or underlaid by a water-bearing portion of the formation.
The well may be completed by employing conventional completion practices such as running a cement casing in the well and forming perforations through the casing and cement sheaths around the casing, thereby forming an open production interval which communicates with the formation.
In the case of a hydrocarbon-bearing formation it is normally desirable to form the open production interval so that it communicates with the hydrocarbon-bearing portion of the formation but does not extend into and communicate with the water-bearing portion. However, the open production interval which is formed in the well may inadvertently communicate with a water-bearing portion which is completed in the same wellbore as the hydrocarbon-bearing portion of the formation.
Even if there is no actual initial fluid communication between the open production interval and the water-bearing portions of the formation, such communication may develop during production of hydrocarbon from the hydrocarbon-bearing portion of the formation. For example, water may be drawn upwardly from the water-bearing portion into the oil-bearing portion about the well. This phenomenon is known as water coning. In the case of water coning, free water is produced in the well which results in a much higher water-to-oil ratio in the production stream than would be the case without the water coning. The higher water-to-oil ratio is undesirable and results in increased operating costs.
Various techniques have been employed to counteract the effects of water coning or water-influx. One such technique involves completing the production well by providing an open production interval which communicates with the hydrocarbon-bearing portion of the formation but that does not communicate with the water-bearing portion which may overlie or underlie the hydrocarbon-bearing portion of the formation. The well may then be produced at a sufficiently low rate that coning of water into the hydrocarbon-bearing formation and about the well is avoided. However, such production rates may be lower than are otherwise desirable.
Other methods which may be employed to reduce the effects of water coning include placing of barriers within the subsurface formations penetrated by wells. One method of preventing water flow into oil wells involves creating a substantially horizontal barrier which extends radially and outwardly from the well into the oil zone of the oil-bearing formation. The barrier is located between the perforations through which oil flows into the well and the zone of the fluid which is to be blocked. The flow of water into the perforations is restricted by the barrier. Portland cement is commonly used as a barrier-forming material. For various reasons, however, substantial water may flow through or around these barriers to the perforations through which the oil is produced into the well. U.S. Pat. No. 3,237,690 to Jacy C. Karp et al. is directed to the creation in an oil zone of a thin, substantially horizontal, highly impermeable barrier of portland cement which is bound by strata of the formation which have had their permeability permanently destroyed by a plugging material.
A method of forming a plastic pancake barrier between an oil and water zone is described in The Oil and Gas Journal, Jul. 11, 1960, Vol. 58, No. 28, p. 71. In accordance with this technique, a low viscosity plastic is squeezed into well perforations at the water-oil contact to form an impermeable pancake or cylinder of plastic at the water-oil contact. The low viscosity plastic was forced at less than fracturing pressure into the pores of the formation.
Still another method of decreasing the ratio of gas and/or water produced along with oil in a well is described in U.S. Pat. No. 3,368,624 issued to Heuer et al. Here a foam plug is formed in situ in the path of the water. This foam plug is formed by injecting a foaming agent into a subterranean formation surrounding a wellbore and placing the well on production. Heuer et al. state that it is preferred that the foaming agent be injected so that it primarily enters the formation near the interface of the water- and oil-producing zones. More preferably, the foaming agent is injected directly into the water zone.
Although several methods have addressed the problem of water coning, none of these methods are directed to preventing water coning or water-influx where the hydrocarbon-bearing formation is water sensitive. Where this formation exists it is inadvisable to block fluid flow with a water-based profile control gel. This is inadvisable because leak-off of water from a water-based gel system could contact and damage an oil- or gas-producing but water-sensitive hydrocarbon zone. Such leak-off is difficult to avoid in the very common circumstances in which the water-sensitive, hydrocarbon-producing zone and the water-influx zone are both communicating with the wellbore via separate sets of perforations. Damage would result because of a catastrophic loss of permeability when the water-sensitive zone is exposed to water.
Therefore, what is needed is a method to prevent water coning or water-influx when the hydrocarbon-bearing formation is water-sensitive.